TWI666195B - Acrylonitrile reactor startup procedure - Google Patents

Abstract

The formation of an explosive mixture during the startup of the acrylonitrile reactor is prevented by including ammonia into the gas charged to the reactor during catalyst warm-up. In addition to the heat generated, this oxidation of ammonia reduces the oxygen content of the gas in the acrylonitrile reactor and in the reactor effluent gas, thereby reducing the risk that an explosive mixture will form in the effluent gas.

Description

Acrylonitrile reactor startup procedure

The present invention relates to an acrylonitrile reactor startup procedure.

Background of the invention

In the commercial manufacture of acrylonitrile, propylene, ammonia and oxygen react together according to the following reaction scheme: CH ₂ = CH-CH ₃ + NH ₃ +3/2 O ₂ → CH ₂ = CH-CN + 3 H ₂ O This procedure, known as ammonia oxidation, is performed in the gas phase at elevated temperatures in the presence of a suitable fluidized bed ammonia oxidation catalyst.

Figure 1 shows a typical ammoxidation reactor used to perform this procedure. As shown here, the reactor 10 includes a reactor housing 12, an air grill 14, a feed sprinkler 16, a cooling coil 18, and a cyclone separator 20. During normal operation, process air is charged into the reactor 10 via an air inlet 22 while a mixture of propylene and ammonia is charged into the reactor 10 via a feed sprayer 16. The flow rate of both is high enough to fluidize the ammonia oxidation catalyst bed 24 in the interior of the reactor, where propylene and ammonia catalyzed ammoxidation to acrylonitrile occurs.

The product gas produced by the reaction flows out of the reactor 10 through a reactor effluent outlet 26. Before doing so, they pass through the cyclone separator 20, The cyclone separator 20 removes any ammonia oxidation catalyst that these gases can entrain for returning to the catalyst bed 24. Ammonia oxidation is extremely exothermic, and therefore the cooling coil 18 is used to recover the residual heat, and therefore the reaction temperature is maintained at an appropriate level.

As discussed further below, one of the early steps in the startup of the acrylonitrile reactor is to preheat the ammoxidation catalyst to an elevated temperature. For this purpose, the start-up heater 28 is provided to heat the process air fed to the air inlet 22 during the catalyst warm-up step.

Propylene and ammonia, and combustible components of the reactor effluent gas (eg, acrylonitrile, unreacted propylene, unreacted ammonia, hydrocyanic acid, acrolein, acrylic acid, and acetonitrile) can form explosive mixtures with oxygen. Therefore, during normal operation, as well as during startup, care must be taken to avoid situations in which an explosion can occur. During normal operation, within the reactor at normal operating temperatures, this is not a problem because the ammoxidation reaction prevents explosions from occurring. Therefore, the reactor 10 is designed and operated so as to allow the only location where the process air contacts propylene and ammonia during normal operation is within the fluidized bed of the ammonia oxidation catalyst 24, and then only when the temperature of the catalyst is high enough to catalyze the ammonia oxidation reaction .

However, during startup and shutdown, the temperature of the ammoxidation catalyst is usually not high enough to prevent explosions. Therefore, different approaches to preventing explosions are usually employed, all of which are first based on the idea of avoiding the formation of explosive mixtures.

In this regard, to be an explosive, a mixture of flammable components and oxygen needs to contain a certain minimum concentration of flammable components, which is called the "lower flammability limit" concentration of the component. In addition, the mixture must contain a certain minimum Oxygen concentration to support the combustion of combustible components is known as the "limiting oxygen concentration" of the mixture. Therefore, in all relevant gas mixtures, the early approach for avoiding explosive mixtures during start-up relied on the "fuel-restricted pathway" or "oxygen-restricted pathway" in which the concentration of flammable components remained below its flammability Lower limit concentration, in which the oxygen concentration remains below its limit oxygen concentration.

For example, in a typical oxygen-limited approach for starting an acrylonitrile reactor, heated process air is used to preheat the catalyst to a suitable elevated temperature. Once this occurs, the heated process air stream is replaced by a heated inert gas stream (typically steam or nitrogen) until the oxygen concentration in the reactor effluent gas drops to a safe level, that is, below the limit of the effluent gas during normal operation The level of oxygen concentration. Only after this has occurred does the flow of propylene and ammonia to the reactor begin. The explosive mixture did not form in the reactor effluent gas because the oxygen concentration in the reactor effluent gas dropped to before flammable components such as acrylonitrile, HCN, unreacted propylene and unreacted ammonia appeared in the effluent Its limiting oxygen concentration is below.

Steam can have adverse effects on the ammoxidation catalyst and the interior of the reactor, and therefore steam is not preferred for this purpose. Although these problems are avoided in the case where nitrogen is used as an inert gas, a large amount of nitrogen is required, which can be prohibitively expensive in many cases. Therefore, the use of an oxygen-limiting route for avoiding explosive gas during startup of an ammoxidation reactor is not often used due to these undesirable characteristics.

In a typical fuel-limited approach for starting an ammoxidation reactor, heated process air is also used to preheat the catalyst to a suitable elevation temperature. Once this temperature is reached, the flow of propylene and ammonia to the reactor begins, but this is only done very slowly. Since these reactants are quickly consumed by the ammoxidation reaction, and also because of their slow flow rate, the concentration of the flammable components in the reactor effluent gas is always kept below its lower flammability limit concentration. Therefore, the concept of this approach is that even if the oxygen concentration in the reactor and the reactor effluent gas is relatively high, as long as the flow rates of propylene and ammonia to the reactor are slow during startup, combustible components appearing in the reactor effluent gas The amount will always be less than its lower flammability limit concentration.

However, the problem with this fuel restriction approach is that once the system reaches normal operating conditions, explosive mixtures in the outflow gas are prevented because the oxygen concentration in the gas is too low, not because the concentration of the flammable components is too low. This means that when using this approach, the system transitions between a fuel-restricted approach and an oxygen-restricted approach to avoid explosive mixtures as the system progresses from startup to normal operation. The problem is that during this transition, the concentration of combustible components in the effluent gas on the one hand, and the concentration of oxygen in the effluent gas, on the other hand, can be quite close to each other in generating an explosive mixture.

In this regard, it should be kept in mind that the lower limit of the flammability of the flammable components in the gas mixture and the corresponding limit oxygen concentration in the gas mixture change inversely relative to each other. That is, if the oxygen concentration of the gas mixture increases, the lower limit of the flammability of the flammable component in the gas mixture decreases, and vice versa. In addition, the flammability range of the flammable component in the gas mixture (that is, the difference between its upper flammability limit and the lower flammability limit) and the difference between the maximum limiting oxygen concentration and the minimum limiting oxygen concentration of the mixture increase as the The temperature increases.

Therefore, when the system transitions between a fuel-restricted pathway used to avoid explosive mixtures during the early start-up period and an oxygen-restricted pathway during the later phase of start-up, they can come to the concentration of combustible components and oxygen in the effluent gas in terms of generating explosive mixtures. Pretty close to each other. Therefore, if this approach is used, not only the precise control of the reaction temperature and the feed rate of propylene and ammonia is required, but even if this precise control is provided, there is still a considerable risk that the effluent gas can become an explosive.

What makes the problem even worse is that the products and by-products of the ammoxidation reaction (eg, acrylonitrile, HCN, acetonitrile, acrylic acid, and acrolein) are also flammable. So even if the effluent gas may not be an explosive relative to propylene or ammonia, it may still be an explosive relative to these products and by-products.

Another problem with this fuel-limited approach is that some of the products and by-products of the ammoxidation reaction (eg, acrolein), although not explosive in the presence of sufficiently low concentrations, are encountered in the reactor effluent gas. The temperature and oxygen concentration reached were still unstable. This instability can lead to a combustion reaction (known as "effluent afterburning") that occurs in the effluent gas, which results in an undesirably high effluent temperature.

Summary of invention

According to the invention, a new procedure is provided for avoiding the formation of an explosive mixture during startup, which is easier and cheaper to execute than similar procedures performed in the past.

The start-up procedure for an acrylonitrile reactor involves oxidizing ammonia Filling the at least one ammonia oxidation reactor; heating the ammonia oxidation catalyst to at least a minimum ammonia oxidation temperature; and introducing ammonia and optional propylene into the reactor, wherein about 0 to about Molar ratio of 0.02 to propylene until the oxygen concentration in the reactor effluent is below the limiting oxygen concentration.

In another aspect, a startup procedure for an acrylonitrile reactor includes charging an ammonia oxidation catalyst into at least one ammonia oxidation reactor; heating the ammonia oxidation catalyst to at least a minimum ammonia oxidation temperature; and charging ammonia and optional propylene to An amount effective to raise the catalyst temperature to a temporary reaction temperature of about 415 ° C to about 425 ° C is introduced into the reactor, wherein propylene is introduced into the reactor in an amount effective to prevent an unstable exothermic reaction.

In one aspect, a process for producing acrylonitrile includes charging an ammonia oxidation catalyst into at least one ammonia oxidation reactor; heating the ammonia oxidation catalyst to at least a minimum ammonia oxidation temperature; and using ammonia effectively for providing in the reactor An amount of oxygen concentration below the limiting oxygen concentration is introduced into the reactor; and propylene is introduced into the reactor in an amount effective for producing acrylonitrile. The procedure may include maintaining a propylene to ammonia ratio of about 0 to about 0.02 until the oxygen concentration in the reactor is below the limiting oxygen concentration.

In another aspect, a process for starting an acrylonitrile reactor containing a freshly packed bismuth molybdate ammoxidation catalyst, a method for reducing the sublimation of molybdenum, the method comprising using propylene and ammonia effectively for The catalyst is introduced into the reactor in an amount ranging from a temporary reaction temperature of about 415 ° C to about 425 ° C to a steady state reaction temperature of about 435 ° C to about 445 ° C over a period of 1 hour to about 400 hours.

In another aspect, the present invention provides a new procedure for heating an air-fluidized ammoxidation catalyst in a manner to prevent the formation of explosive gas mixtures during startup of an acrylonitrile reactor, the procedure comprising: (a) fluidizing the air The ammonia oxidation catalyst is preheated to an ammonia activity temperature, which is high enough for the catalyst to oxidize the catalytic ammonia to nitrogen and water. (B) Thereafter, the ammonia in the gas is charged into the reactor to generate the ammonia for the catalyst through the catalytic oxidation of ammonia. Preheated additional heat, wherein the ammonia stream charged into the reactor is sufficient to reduce the oxygen concentration in the reactor effluent gas to below the limit oxygen concentration of the effluent gas, that is, so low that the effluent gas due to oxygen It is not the concentration of explosives any more, and (c) delays the feeding of propylene to the ammoxidation catalyst until the oxygen concentration in the reactor effluent gas decreases below its limit oxygen concentration.

Advantageously, the addition of ammonia is started at step (b) when the temperature of the ammoxidation catalyst reaches about 380 ° C, about 365 ° C or even about 350 ° C. In addition, it is also preferred that once the ammonia addition is started, the amount of ammonia charged into the reactor is sufficient to reduce the oxygen concentration in the reactor effluent gas to less than 10 vol.% Or even 8 vol before the propylene feed to the reactor begins. .%the following.

In addition, the preheating of the ammoxidation catalyst in step (a) is preferably performed using a direct-fired inline heater rather than an indirect-fired heater commonly used for this purpose, as this further significantly reduces the reactor and reaction The oxygen concentration in the effluent gas from the reactor.

In addition to the above ammonia-assisted catalyst heating program, the present invention also provides a new sprayer purification for avoiding clogging or contamination of the sprayer during startup of a reactor using the ammonia-assisted catalyst heating program. A procedure which includes purifying the sprayer with air during the initial phase of startup, and then changing the gas used to purify the sprayer from air to nitrogen shortly before ammonia feed to the reactor begins.

10‧‧‧ Reactor

12‧‧‧ shell

14‧‧‧air grill

16‧‧‧Sprayer

18‧‧‧ cooling coil

20‧‧‧ cyclone separator

22‧‧‧air inlet

24‧‧‧ catalyst bed / ammoxidation catalyst

26‧‧‧Outflow

28‧‧‧ heater

Figure 1 shows a typical ammoxidation reactor for performing an ammoxidation process.

detailed description

The start-up of a commercial fluidized bed acrylonitrile reactor begins with the ammonia oxidation catalyst in a stable (non-fluidized) state resting on top of the air grid 14 of the reactor. In one aspect, a single reactor can be used, in another aspect, more than one reactor can be used, and in another aspect, two reactors can be used. The reactor effluent can be combined at a later point in the process.

The first step of a typical reactor start-up procedure involves purging the feed sprayer with nitrogen, that is, filling the feed sprayer with nitrogen at a flow rate sufficient to prevent the fluidized ammonia oxidation catalyst from entering or clogging the sprayer. At the same time or shortly thereafter, the heated process air is fed to the air inlet 22 at a flow rate sufficient to cause fluidization of the ammonia oxidation catalyst. This heating step continues until the temperature of the ammoxidation catalyst reaches or slightly exceeds its minimum ammoxidation reaction temperature, which typically takes 8 to 16 hours, depending mainly on the size of the reactor. Once this temperature is reached, the system becomes heated by terminating the heating of the incoming process air, replacing the nitrogen charged via the feed sprayer with a mixture of propylene and ammonia, and adjusting the incoming process air flow rate to its normal operating value. Normal operation. To prevent the formation of explosive mixtures, Additional steps described in the scene section.

According to the present invention, a fuel restriction approach is also used to avoid the formation of explosive gas mixtures in the reactor effluent gas during startup. However, the fuel restriction pathway of the present invention differs from the fuel restriction pathway described in the background section of this document in that the ammonia to feed sprayer flow is once the temperature of the ammonia oxidation catalyst is high enough to catalyze the oxidation of ammonia to nitrogen and water ( Its "ammonia activity temperature") is started shortly after or shortly after, rather than simultaneously with the propylene feed as occurs in conventional practice. Therefore, before the propylene is fed to the reactor, the oxygen concentration in the reactor effluent gas can be reduced to such a low level that the gas is no longer an explosive level due to insufficient oxygen concentration.

At a given oxygen concentration, the lower limit of flammability of ammonia is significantly higher than the lower limit of flammability of propylene. In other words, for a given oxygen concentration, the gas mixture can tolerate more ammonia than propylene before becoming explosive. Therefore, according to the present invention, the feeding of ammonia into the reactor starts at an earlier stage than the start-up in other cases, and in addition, before the start of feeding of propylene. Although the oxygen concentration in the effluent gas is higher at these earlier stages of startup than the subsequent phases, this is not a significant issue because the effluent gas can tolerate ammonia concentrations greater than propylene before becoming explosive.

Therefore, the earlier introduction of this ammonia was used to cause the oxygen concentration of the effluent gas to drop to a safe level (ie, below its limit oxygen concentration during normal operation) before feeding propylene to the reactor. The limiting oxygen concentration of the propylene mixture in the air at elevated temperatures (e.g., ~ 440 ° C) experienced by the effluent gas during normal operation is not accurately known, but is estimated to be between ~ 8vol.% To ~ 10vol.% . Therefore, according to the present invention, the propylene is fed to the reactor Before beginning, the early introduction of ammonia continued until the oxygen concentration of the effluent gas dropped to about 10 vol% or less, on the other hand, about 9 vol% or less, on the other hand, about 8 vol% or less, In one aspect, it is about 7 vol% or less, and in another aspect, it is about 6 vol% or less.

The practical effect of this approach is that the risk that an explosive mixture will form in the reactor effluent gas when the system transitions from a fuel-limited mode during startup to an oxygen-limited mode during normal operation is substantially completely eliminated. This is because the target oxygen concentration of 8 vol.% Or less is lower than the limit oxygen concentration of the propylene / oxygen mixture at the elevated temperature that the effluent gas will experience, while the preferred target oxygen concentration of about 6 vol.% To 7 vol.% Is far Below this limit oxygen concentration. Therefore, by delaying the introduction of propylene until the oxygen content of the system is so low, a propylene explosion cannot occur due to the lack of oxygen, regardless of the amount of propylene that the effluent gas can eventually contain. On the other hand, a small amount of propylene can be introduced without any delay. In this regard, a propylene to ammonia ratio of about 0 to about 0.02 is maintained until the oxygen concentration in the reactor is below the limiting oxygen concentration. On the other hand, a ratio of propylene to ammonia of about 0.001 to about 0.02 is maintained, on the other hand is about 0.005 to about 0.02, on the other hand is about 0.01 to about 0.02, and on the other hand is about 0.015 to about 0.02.

In a preferred embodiment of the present invention, a direct combustion inline heater is used to heat the process air fed to the reactor for preheating the catalyst. In this context, “direct combustion inline heater” means a combustion stove configured such that the combustion gas generated by the stove is included in the heating process air generated by the stove and fed to the reactor 10. Direct-fired inline heater and start-up period The difference between indirect combustion heaters commonly used to heat process air is that indirect combustion heaters discharge their combustion products and discard them, rather than combining them with the heated process air they produce.

When the indirect combustion heater heats the process air for catalyst preheating, the process air and therefore the air in the reactor and the reactor effluent gas have the same oxygen concentration as normal air, ie, ~ 21vol.%. In contrast, the heating process air produced by a direct combustion inline heater contains only ~ 18 vol.% Oxygen. Therefore, when the direct combustion on-line heater heats the process air for catalyst preheating, the air in the reactor and in the reactor effluent gas has an oxygen concentration of only ~ 18 vol.% At the start of startup. This in turn means that when the system transitions between the fuel-restricted pathway in the early stages of startup and the oxygen-restricted pathway in the later stages of startup, if a direct-fired inline heater is used instead of an indirect-fired heater, the transition is reduced. ~ 3vol.% Oxygen started. Since the limiting oxygen concentration of the reactor effluent gas during normal operation is higher than 8 vol.%, A decrease in the initial oxygen concentration from ~ 21 vol.% To ~ 18 vol.% To ~ 3 vol.% Represents the oxygen concentration during this transition period The amount of 23% (3 / (21-8)) must be reduced to avoid the formation of explosive mixtures in the effluent gas.

The practical effect of this reduction is not only that the limiting oxygen concentration of the effluent gas is reached faster than in other cases, but also that the concentration of the flammable components in the effluent gas has not reached its lower flammability limit concentration as in other cases. Therefore, the reaction temperature and the feed rate of propylene and ammonia during this critical transition period need not be controlled as precisely as previously required to ensure that the risk of explosion in the effluent gas is avoided.

According to another feature of the invention, air is used instead of the nitrogen gas conventionally used for this purpose to purify the feed sprinkler 16 during the initial phase of startup. Then, shortly before the start of ammonia feed to the reactor, the gas used to purify the sprinkler changed from air to nitrogen. In this context, "shortly before" will be understood to mean within 30 minutes before the start of ammonia feeding. It is also envisaged that the change of sprayer feed from air to nitrogen occurs within 20 minutes, 10 minutes before, or even 5 minutes before the gas used to purify the sprinkler changes from air to nitrogen. The advantage of this approach over the conventional practice of nitrogen for the entire sprinkler purification program is that a significant amount of nitrogen is saved.

An example of a detailed ammonia oxidation reactor startup procedure using the principles of the present invention is described below:

Pre-launch

Prior to start-up, all downstream and auxiliary equipment (e.g. quenching sections, absorbers, recovery towers, vaporizers, steam systems, utilities, etc.) and all required reactor instruments (e.g., reactor temperature sensors, The flu detector and the oxygen analyzer for the reactor effluent) were ready for operation. In addition, a fluidized bed ammoxidation catalyst is charged into the reactor and is in a stable (non-fluidized) state and stays on the air grill 14. The process air feed compressor is then started and set to exhaust to the atmosphere.

Sprayer purification

A stream of nitrogen or air (if desired) is introduced into the sprinkler 16 to prevent the fluidized catalyst from clogging or otherwise fouling the sprinkler.

Formation of air flow to the reactor

Next, the ammonia oxidation catalyst 24 in the reactor 10 starts to pass slowly The reactor air flow controller (not shown) is turned on to fluidize, and then the process air flow via the startup heater 28, the reactor 10, and the associated downstream equipment (not shown) is increased. Generally, the process air flow rate will increase until it reaches normal operating conditions. In this context, "normal operating state" means the state experienced in the reactor 10 after startup is complete and the ammoxidation reaction proceeds to a normal steady state state.

Touch start heater

After the process air flow has been formed to the reactor 10, the start heater 28 is started and operated so that the temperature of the process air flowing out of the heater is higher than the minimum ammonia oxidation temperature of the ammonia oxidation catalyst. In practice, the temperature of the outflow air start heater 28 will be operated to achieve the highest process air temperature possible, typically around 480 ° C to 500 ° C, as this allows the catalyst to be warmed up as quickly as possible, which typically depends on the reactor Size and it takes about 8 to 12 hours.

Although the start-up heater 28 may be an indirect-fired heater, a direct-fired type of heater that is wired for the above-mentioned reasons is desirably used for this purpose, that is, because such a heater will bleed gas from the reactor at the start of the start-up The oxygen concentration in the solution is reduced to ~ 18 vol.%, As opposed to the usual ~ 21 vol.% That would be achieved if an indirect combustion heater was used.

Preheat catalyst

The heated process air stream entering the reactor 10 continues at least until the temperature of the ammonia oxidation catalyst reaches its lowest ammonia oxidation temperature, ie, the lowest temperature at which the ammonia oxidation catalyst is capable of catalyzing ammonia oxidation to nitrogen and water, which is typically About 180 ° C to 200 ° C. However, typically, heating the process air will For preheating an ammonia oxidation catalyst to a higher temperature, for example, to at least 350 ° C, at least 375 ° C, or even at least 390 ° C, as this reduces the amount of ammonia required in the subsequent ammonia combustion step described below, This is more economical.

Preparation for initial ammonia introduction

Shortly before the ammonia is introduced into the reactor, if desired, the flow rate of the process air may be slightly reduced because this reduces the amount of ammonia required in this step and the subsequent ammonia introduction step. It is envisaged that a decrease in the flow rate that occurs within 30 minutes, 20 minutes, or even 10 minutes before the first introduction of ammonia into the reactor is 30% to 95%, 40% of the process air flow rate during normal operating conditions Reduction in process air flow rate to 85% or even 50% to 75%.

In addition to the reduction of the optional process air flow rate, if air is used as a purge gas to prevent contamination of the sprinkler 16 as described above in connection with an embodiment of the present invention, the air passing through the sprinkler 16 The flow needs to be stopped and replaced with nitrogen as purge gas before ammonia introduction begins. If the air flow rate is reduced, it remains at its reduced flow rate during the initial and continued introduction of ammonia feed, while the oxygen concentration of the effluent gas from the reactor decreases. When the propylene feed is first introduced and then increased in a stepwise manner, the air flow rate and ammonia flow rate are also increased in a stepwise manner until the propylene, air, and ammonia feed rates reach their normal final values.

Initial ammonia introduction

Once all the air has been purged from the sprayer 16, the flow of ammonia into the reactor 10 via the feed sprayer 16 begins and increases to a suitable level, preferably stepwise, to facilitate temperature control of the reactor. As mentioned above, this leads The ammonia oxidation catalyst 24 in the reactor 10 catalytically oxidizes the ammonia feed to nitrogen and water. One consequence of this oxidation reaction is that the amount of oxygen in the reactor 10 and therefore the reactor effluent gas leaving the reactor is significantly reduced. Another consequence of this oxidation reaction is that a significant amount of heat is generated, which helps the catalyst to warm up, thereby reducing the amount of propylene required for this purpose. The cooling coil 18 can be put into use to control the temperature of the ammonia oxidation catalyst in the reactor 10 as needed. Although a slight rise in temperature within the reactor may be allowed, it is desirable to maintain it at or near the reactor temperature encountered during normal operation, which is typically about 350 ° C to 480 ° C.

In this particular example of initial ammonia introduction, the flow rate of the ammonia feed may first be set at a low level, for example, at a value that gives a molar feed ratio of 13 to 15 air to ammonia. At this initial low ammonia feed rate, the ammonia concentration in the reactor effluent gas will be about 6% to 7%, which is well below its lower flammability limit concentration in air. Once ammonia oxidation begins, as the oxygen concentration in the effluent gas from the reactor is clear, the ammonia feed rate can be further increased.

If desired, the incoming process air, which is heated by means of the start-up heater 28, can be stopped at the beginning or shortly after the feed of ammonia to the reactor. However, as described further below, it is more desirable to keep the heater operation on until the propylene introduced into the reactor reaches its final desired rate, as this can significantly reduce the cost of ammonia required for catalyst preheating.

Continue ammonia feed introduction

The flow rate of ammonia into the reactor 10 via the sprayer 16 may increase suddenly or continuously. Desirably, as described above, for better reactor temperature control, the ammonia flow rate is gradually increased. The flow rate of the ammonia feed The flow rate of the process air is increased until the oxygen concentration in the effluent gas of the reactor falls below the target value of 8 vol.%, Which is lower than the limit oxygen concentration of propylene in the effluent gas. Preferably, the flow rate of the ammonia feed is increased until the oxygen concentration in the reactor effluent gas drops to ~ 6 vol.% To ~ 7 vol.%. In practice, the oxygen concentration in the ~ 7vol.% Reactor effluent gas corresponds to a volumetric feed ratio of air to ammonia of about 5.

Propylene feed introduction

When the concentration of oxygen in the reactor effluent gas is reduced to a desired value of less than 8 vol.%, Preferably ~ 6 vol.% To ~ 7 vol.%, The flow of propylene into the reactor 10 via the sprayer 16 begins. Thereafter, the propylene flow rate is desirably increased in a stepwise manner to achieve precise reactor temperature control until the final desired propylene flow rate is achieved. At this time, if it has not been completed before, the heater 28 may be stopped.

In one aspect, a startup procedure for an acrylonitrile reactor includes charging an ammoxidation catalyst into at least one ammoxidation reactor. The procedure includes heating the ammonia oxidation catalyst to at least a minimum ammonia oxidation temperature. In this aspect, when the ammonia oxidation catalyst reaches about 350 ° C or higher (on the other hand, about 350 ° C to about 480 ° C, on the other hand, about 375 ° C to about 450 ° C, and on the other hand, (> 400 ° C to about 425 ° C) at the lowest ammonia oxidation temperature, ammonia and optional propylene are introduced into the reactor. On the other hand, a molar ratio of propylene to ammonia of about 0 to about 0.02 is maintained in the reactor effluent until the oxygen concentration in the reactor effluent is below the limit oxygen concentration, and on the other hand, about 0 to A ratio of about 0.01, and on the other hand, a ratio of about 0.01 to about 0.02 is maintained.

In another aspect, the procedure includes maintaining a molar ratio of propylene to ammonia of about 0 in the reactor effluent until the oxygen concentration in the reactor effluent is below the limiting oxygen concentration. In another aspect, the limiting oxygen concentration in the reactor effluent can be about 10 vol% or less, in another aspect, about 9 vol% or less, in another aspect, about 8 vol% or less, in another In terms of, it is about 7 vol% or less. On the other hand, when the oxygen concentration in the reactor effluent is about 12 vol% or more, the ratio of propylene to ammonia in the reactor effluent is about 0. In a related aspect, when the oxygen concentration in the reactor effluent is about 7 vol% or less, the ratio of propylene to ammonia in the reactor effluent is about 0.02. In some aspects, the oxygen concentration in the reactor effluent can be maintained at a concentration of at least about 0.5 vol% or greater, in another aspect, from about 0.5 vol% to about 7 vol%, and in another aspect, about 0.5 vol % To about 1.5 vol%, and in another aspect, about 0.5 vol% to about 2 vol%. The oxygen concentration in the reactor effluent can be measured by any known method, such as, for example, by a continuous in-line oxygen meter.

Acrylonitrile plant with multiple reactors

In a typical commercial acrylonitrile plant, the hot reactor effluent gases leaving reactor 10 first enter a quench tower, where they are sprayed with acidified water. This not only reduces the temperature of the effluent gas to a safe and manageable level, but also neutralizes any unreacted ammonia that may still be present. The cooled reactor effluent gas, which now removes its unreacted ammonia, is then transferred to an absorption tower, where they come in contact with an additional amount of water, which makes the water-soluble components of the gas by absorption of the water-soluble components in the aqueous phase (e.g., acrylonitrile, HCN and acetonitrile) with a non-water soluble components _{(e.g., N 2, CO 2, CO} , propane, propylene) separation. Next, the liquid bottom sediment of the absorption tower is sent to a recovery tower, where crude acrylonitrile and HCN are separated from acetonitrile by distillation.

It is not uncommon for large commercial acrylonitrile plants to include more than one (two, three, or even more) separate ammoxidation reactors, and the reactors share one or more common "back-ends" for recovery and Purify the reaction products of acrylonitrile, HCN and acetonitrile. In some of these plants, each ammoxidation reactor will have its own dedicated quench tower, with the cooled reactor effluent gas leaving these towers being transferred to a single common absorption tower. In others of these plants, the hot reactor effluent gas leaving the ammoxidation reactor is transferred to a common quench tower.

As mentioned above, the improved reactor start-up procedure of the present invention can also be used in this type of commercial acrylonitrile plant by separately starting a series of independent reactors, with the condition that it comes from the second reactor and subsequent reactions The effluent gas from the reactor is not sent to a common back-end equipment (ie, a common absorption tower or a common quenching tower, as the case may be) until its oxygen concentration drops to 8% or less, preferably 7% or less. Instead, the effluent gases from the second and subsequent reactors are discharged, incinerated or otherwise discharged for disposal until such time that their oxygen concentration decreases to these levels. In the alternative, the second reactor and subsequent reactors can be started with nitrogen (N ₂ ) added to the second reactor feed and subsequent reactor feed as such a diluent in order to remove The oxygen concentration in the effluent of the second and subsequent reactors is maintained at 8% or less, preferably 7% or less.

In one aspect, a startup procedure for an acrylonitrile reactor includes charging an ammonia oxidation catalyst into at least one ammonia oxidation reaction reactor, and charging The ammoxidation catalyst is heated at least to a minimum ammoxidation temperature as described herein. The procedure also includes introducing ammonia and optional propylene into the reactor in an amount effective to increase the catalyst temperature to a temporary reaction temperature of about 415 ° C to about 425 ° C, wherein the propylene is effective to prevent unstable heat generation The amount of the reaction was introduced into the reactor. In this aspect, a temporary reaction temperature of about 415 ° C to about 425 ° C is reached in about 5 hours or less after propylene is introduced into the reactor. An unstable exothermic reaction refers to a reaction in which the temperature cannot be maintained in a range of about 415 ° C to about 425 ° C for a desired amount of time and uncontrollably exceeds this range.

In another aspect, the procedure can further include introducing propylene into the reactor in an amount effective to increase the temporary catalyst temperature to a steady state reaction temperature of about 435 ° C to about 445 ° C. In this aspect, a temperature of about 435 ° C is reached about 200 hours or more after the introduction of propylene into the reactor, and on the other hand, a temperature of about 440 ° C is reached about 250 hours or more after the introduction of propylene into the reactor, And in another aspect, a temperature of about 435 ° C to about 445 ° C is reached about 1 hour to about 3 hours after the propylene is introduced into the reactor.

Start with fresh catalyst

A typical commercial ammoxidation reactor operates with an ammoxidation catalyst maintained at a temperature of about 435 ° C to 445 ° C (eg, 440 ° C). Therefore, when the starting acrylonitrile reactor contains an equilibrium catalyst, that is, an ammonia oxidation catalyst that has been used long enough for its composition to remain substantially constant for a certain period of time, the reactor temperature is brought to this level (about 435 ° C to 445 ° C) without delay. That is, once the ammoxidation reaction starts, propylene and ammonia to ammonia The feed of the oxidation catalyst rapidly increases to the level required to achieve steady state operation at these temperatures, which typically takes from 1 hour to 3 hours.

However, fresh or new bismuth molybdate ammoxidation catalysts (ie, catalysts that are unbalanced catalysts) are known to undergo chemical changes at startup (ie, exposure to reactor operating temperature), where their molybdenum content Part of it was lost due to sublimation. Although this phenomenon does not adversely affect the operation of the catalyst to any significant extent, the metal molybdenum sublimated from the catalyst is usually condensed on the cooling coils of the ammoxidation reactor, which can cause a variety of operational problems and equipment problems.

According to another feature of the invention, the normal procedure for starting a commercial ammonia oxidation reactor containing a new batch of bismuth molybdate type ammonia oxidation catalysts is to pass a final steady state ammonia oxidation catalyst that will reach about 435 ° C to about 445 ° C. The reaction temperature was delayed by about two weeks to modify. According to this approach, the flow rates of propylene and ammonia to the ammoxidation catalyst are first increased in a conventional manner for achieving a rapid increase in the reaction temperature. However, in this case, when the ammoxidation catalyst reaches a temporary reaction temperature of about 415 ° C to 425 ° C (for example, 420 ° C), this rapid increase in reaction temperature is interrupted. At this time, the flow rates of propylene and ammonia are changed so that the temperature of the ammoxidation catalyst is only very gradually over a period of about two weeks (that is, in a period of about 275 to 400 hours, and more commonly about 325 Hours to 350 hours) to its final steady state value of about 435 ° C to 445 ° C (eg, 440 ° C). This approach has been found to significantly reduce the rate of release of molybdenum from the catalyst and therefore the rate at which metallic molybdenum condenses on the reactor cooling coils.

In another aspect, a temporary temperature between about 415 ° C and about 425 ° C The reaction temperature was reached within about 5 hours of the introduction of propylene into the reactor. In another aspect, a reaction temperature between about 435 ° C and about 445 ° C is reached within about one hour to about 3 hours of the introduction of propylene into the reactor. On the other hand, when the temporary reaction temperature is reached, the propylene feed rate is gradually increased so that a reaction temperature of about 435 ° C or more is reached about 200 hours or more after the propylene is introduced into the reactor. In another aspect, a temperature of about 440 ° C or greater is reached about 250 hours or more after the propylene is introduced into the reactor.

In another related aspect, the molar ratio of air to propylene remains above the steady state air to propylene molar ratio after the propylene is introduced into the reactor for about 3 hours to about 96 hours, and in another aspect, about 10 hours to about 90 hours. Hours, on the other hand, from about 30 hours to about 70 hours, on the other hand, and from about 40 hours to about 60 hours. In another aspect, the molar ratio of air to propylene is maintained above the steady state air to propylene molar ratio after introducing propylene into the reactor at a molar ratio of propylene to ammonia above about 0.02. Hours, on the other hand, from about 10 hours to about 90 hours, on the other hand, from about 30 hours to about 70 hours, on the other hand, and from about 40 hours to about 60 hours.

On the other hand, the ammonia breakthrough in the reactor effluent is above the molar ratio of propylene to ammonia of about 0.02, and after introducing propylene into the reactor, it stays within the steady-state ammonia breakthrough range. In this aspect, the steady-state ammonia breakthrough ranges from about 6 mol% to about 9 mol% of the ammonia feed. On the other hand, after introducing propylene into the reactor above a molar ratio of propylene to ammonia of about 0.02, the molar ratio of ammonia to propylene is maintained to be greater than the molar ratio of steady state ammonia to propylene by about 0.05 to about 0.15. The interval is about 3 hours to about 96 hours. In another related party On the other hand, reactor ammonia penetration is maintained based on the sulfuric acid feed rate required to maintain a constant pH in the quenching system.

In another aspect, the reactor effluent is maintained within a steady state excess oxygen concentration range after introducing propylene into the reactor above a molar ratio of propylene to ammonia of approximately 0.02, wherein the steady state excess oxygen concentration range is approximately 0.05 mol % To about 1.5 mol%.

In another aspect, the molar ratio of air to propylene remains between about 0.5 to about 1.5 greater than the molar ratio of steady state air to propylene until propylene is introduced into the reaction above a molar ratio of propylene to ammonia of about 0.02. About 3 hours to about 96 hours after the device. The reactor excess oxygen concentration can be maintained based on the measurement of the reactor effluent by a continuous in-line oxygen meter.

On the other hand, certain types of catalysts may require a certain level of oxygen to prevent reduction. In this aspect, the oxygen concentration can be maintained in the reactor effluent between about 0.5 vol% to about 7 vol%, in another aspect, about 0.5 vol% to about 6 vol%, and in another aspect, about 0.5 vol% to About 5vol%, in another aspect, about 0.5vol% to about 4vol%, in another aspect, about 0.5vol% to about 3vol%, in another aspect, about 0.5vol% to about 2vol%, and in another In terms of about 0.5 vol% to about 1.5 vol%.

As can be seen from the foregoing description, the improved ammonia oxidation reactor start-up procedure of the present invention provides a new way to start the reactor, which is not only by better avoiding explosive gas mixtures (especially in the reactor effluent gas). A safer operation is achieved, and the operation is very simple and cheap.

Although only a few specific implementations of the invention have been described above Examples, but it should be apparent that many modifications can be made without departing from the spirit and scope of the invention. All such modifications are intended to be included within the scope of this invention, which is limited only by the scope of the following patent applications.

Claims (28)

A startup procedure for an acrylonitrile reactor, comprising: charging an ammonia oxidation catalyst into at least one ammonia oxidation reactor; heating the ammonia oxidation catalyst to at least a minimum ammonia oxidation temperature; and introducing ammonia and optional propylene In the reactor, wherein the molar ratio of propylene to ammonia in the reactor effluent is maintained from 0 to 0.02 until the oxygen concentration in the reactor effluent is 10 vol% or less. The process of claim 1, wherein the molar ratio of propylene to ammonia in the reactor effluent is maintained at 0 until the oxygen concentration in the reactor effluent is 10 vol% or less. The process as claimed in claim 1, wherein said oxygen concentration in said reactor effluent is 9 vol% or less. The process of claim 3, wherein the oxygen concentration in the reactor effluent is 8 vol% or less. The process of claim 4, wherein the oxygen concentration in the reactor effluent is 7 vol% or less. The process as claimed in claim 1, wherein the ratio of propylene to ammonia in the effluent of the reactor is 0 when the oxygen concentration in the reactor is 12 vol% or more. The process of claim 1, wherein when the oxygen concentration in the reactor is 7 vol% or less, the ratio of propylene to ammonia in the reactor effluent is 0.02 or more. The process as claimed in claim 1, wherein said oxygen in the reactor effluent is 0.5 vol% to 7 vol%. The process as claimed in claim 1, wherein when the ammonia oxidation catalyst has a temperature of 350 ° C or more, ammonia is introduced into the reactor. The procedure of claim 1, wherein the oxygen concentration in the reactor effluent is measured by a continuous on-line oxygen meter. A startup procedure for an acrylonitrile reactor, comprising: charging an ammonia oxidation catalyst into at least one ammonia oxidation reactor; heating the ammonia oxidation catalyst to at least a minimum ammonia oxidation temperature; and charging ammonia and optional propylene to An amount effective to increase the catalyst temperature to a temporary reaction temperature of 415 ° C to 425 ° C is introduced into the reactor, wherein propylene is introduced into the reactor in an amount effective to prevent an unstable exothermic reaction; and wherein A molar ratio of propylene to ammonia of 0 to 0.02 is maintained in the reactor effluent. The process according to claim 11, wherein the ammonia is introduced when the ammonia oxidation catalyst has a temperature of 350 ° C or higher. The process of claim 11, wherein the temporary reaction temperature of 415 ° C. to 425 ° C. is reached 5 hours or less after the introduction of propylene into the reactor. The process of claim 11, further comprising introducing propylene into the reactor in an amount effective to increase the temperature of the temporary catalyst to a steady-state reaction temperature of 435 ° C to 445 ° C. The process of claim 14, wherein a temperature of 435 ° C is reached 200 hours or more after the introduction of propylene into the reactor. The process of claim 14, wherein a temperature of 440 ° C is reached 250 hours or more after the introduction of propylene into the reactor. The process of claim 14, wherein a temperature of 435 ° C to 445 ° C is reached from 1 hour to 3 hours after the introduction of propylene into the reactor. A process for producing acrylonitrile, said process comprising: charging an ammonia oxidation catalyst into at least one ammonia oxidation reactor; heating said ammonia oxidation catalyst to at least a minimum ammonia oxidation temperature; and using ammonia and propylene to effectively use An amount of oxygen concentration of 10 vol% or less in the reactor effluent is introduced into the reactor; and propylene is introduced into the reactor in an amount effective to produce acrylonitrile; wherein, in the The molar ratio of propylene to ammonia in the reactor effluent was maintained from 0 to 0.02 until the oxygen concentration in the reactor was 10 vol% or less. The process of claim 18, wherein the ammonia is introduced when the ammonia oxidation catalyst has a temperature of 350 ° C or higher. The process of claim 19, wherein a molar ratio of propylene to ammonia of 0 is maintained in the reactor effluent until the oxygen concentration in the reactor effluent is 10 vol% or less. The process of claim 20, wherein the oxygen concentration in the reactor effluent is 9 vol% or less. The process of claim 21, wherein the oxygen concentration in the reactor effluent is 8 vol% or less. The process of claim 22, wherein the oxygen concentration in the reactor effluent is 7 vol% or less. The process of claim 18, wherein heating the ammonia oxidation catalyst to a minimum ammonia oxidation temperature is performed using a direct combustion inline heater. The process of claim 18, wherein the reactor includes a feed sprayer for receiving the propylene and ammonia reactants of the ammonia oxidation reaction, and further wherein a fluidized ammonia oxidation catalyst is used during the startup of the reactor. Clogging of the feed sprayer is prevented by a sprayer purge procedure that includes using the air to purge the feed sprayer during the initial phase of startup, and then feeding ammonia to the reaction The gas used to purify the sprayer was changed from air to nitrogen before the sprayer started. The process of claim 18, wherein the acrylonitrile reactor is part of an acrylonitrile plant, the acrylonitrile plant includes a first acrylonitrile reactor and a second acrylonitrile reactor, and An acrylonitrile reactor and a second acrylonitrile reactor to generate acrylonitrile recovery and purification section of the reactor effluent gas recovery, wherein the first acrylonitrile reactor and the second acrylonitrile react Both reactors are started after the procedure of claim 1, and further wherein the reactor effluent gas generated by the second acrylonitrile reactor is not fed to the recovery and purification section of the acrylonitrile plant until Its oxygen concentration is reduced to 8 vol% or less. The process of claim 26, wherein the reactor effluent gas generated by the first acrylonitrile reactor is sent to the recovery and purification section, and wherein the gas generated by the second acrylonitrile reactor is The reactor effluent gas has an oxygen concentration of greater than 6 vol%. The process of claim 27, wherein the reactor effluent gas generated by the first acrylonitrile reactor is fed to the recovery and purification section, and wherein the gas generated by the second acrylonitrile reactor is The reactor effluent gas has an oxygen concentration of greater than 8 vol%.